1. Field of the Invention
This invention relates to processes for recrystallizing sugars and products thereof. Specifically, this invention relates to a low moisture process for high shear mixing of a powdered sugar and an adjuvant such as an invert sugar or molasses and the products thereof.
2. Description of the Background Art
A variety of sucrose crystalline transformation processes are known. These procedures concentrate a sugar syrup by evaporative boiling until the syrup becomes supersaturated. The feed stock sugar syrup is concentrated to an excess of 90 percent solids. Nucleation of crystals is then induced by mechanical means. The initiation of nucleation and the cooling of the supersaturated sugar syrup permits crystallization to proceed very quickly. In order to prevent the formation of a solid mass of sugar crystals, it is necessary to agitate the sugar syrup during crystallization. Agitation is normally achieved by stirring the crystallizing sugar mass with paddles. Such paddles often provide the required nucleation for these transformation processes. Sucrose crystallization is exothermic, because of the positive heat of crystallization, and the liberated heat can be sufficient to evaporate most of the remaining water in the sugar syrup. The resulting "transformed" or crystallized sugar can then be dried to less than 1 percent moisture, milled, and sized in separate operations. The individual grains of transformed sugar are clusters of microsized crystals of sucrose and contain all of the soluble non-sugar solids (i.e., impurities) present in the original syrup.
U.S. Pat. No. 3,365,331 to Miller et. al. discloses a sugar process and product. The invention of this patent is a sugar crystalline transformation process for making dry sugar products comprising agglomerates of fondant-size sucrose crystals. Fondant-size sucrose crystals are from 3 to 50 microns. A liquid sucrose feed stock containing 3 to 15 percent by weight soluble nonsucrose solids, based on the solids content of the syrup, is vacuum concentrated to 91 to 97 percent solids and maintained at a temperature of 120.degree. C. to 130.degree. C. so as to avoid premature crystallization. The supersaturated feed sugar syrup is then transformed in a beater-crystallizer device. A beater-crystallizer device is a means for providing a high-intensity combination of impact, turbulence, and shear to nucleate and rapidly crystallize the sugar syrup. The advantages of this beater-crystallizer device are attainable only if a forced and measured co-current air flow is used to, (1) remove the moisture released as water vapor, and (2) cool the product during the impact beating. The paddle velocity at the center of the blade area must be in the range of 50 to 85 feet per second and the air flow must be in the range of 9 to 20 cubic feet of air per pound of sugar solids throughput. The average particle retention time in the beater-crystallizer device is 10 to 60 seconds. The emerging product has a moisture content of 1.5 to 2.5 percent and must be further dried, ground, and sifted.
A disadvantage of the agglomeration processes, such as that of the Miller et al. '331 patent, is the effect of the humidity level of the forced air flow within the impactor on crystallization efficiency. In hot, humid regions, such as the Louisiana delta, Florida, and other tropical climates, it is almost impossible for a beater-crystallizer device to function properly unless the air is first dried. Such drying is undeniably expensive because of the amount of air flow required in these processes. Also, a significant amount of energy is required to reduce the size of the crystallized sugar mass in the crystallizing zone of the impactor.
U.S. Pat. No. 3,972,725 to Nicol discloses a production of crystalline sugar. A feed sugar syrup in this invention is prepared to the same specifications as those cited in the Miller '331 patent. The hot, supersaturated syrup is subjected to a shear force having a velocity gradient of at least 5000 cm/sec/cm and, preferably, at least 20,000 cm/sec/cm by passing the supersaturated syrup through either a colloid mill or a homogenizer to induce nucleation. The residence time of the feed sugar syrup in this operation is no greater than one second.
U.S. Pat. No. 4,640,717 to Shukla et. al. discloses a sugar process. The process of this patent is related to the process of the Nicol '725 patent, but a twin-screw extruder provides shear. The twin-screw extruder provides a progressive nucleation of the feed sugar syrup with an average retention time below 25 seconds for the feed sugar syrup. The nucleated syrup in this process, as well as in the process of the Nicol '725 patent, is immediately discharged before substantial, if any, crystallization has occurred. The nucleated syrup is expelled in a thin layer onto a moving conveyor belt where essentially all of the exothermic crystallization occurs quiescently and without agitation. The crystalline transformation process normally occurs within about five minutes as the product cools and hardens on the conveyor belt. The product is scraped from the conveyor belt and commonly has a moisture content of about 4 percent by weight. The transformed sugar crystals must be further dried, particulated, and screened.
A disadvantage of transformation procedures, such as the Nicol '725 and the Shukla et al. '717, is that the crystallization step must occur "quiescently." This disadvantage does not allow for the use of agitation, which is often a desired procedure in the development of very fine crystals at a rapid rate. In addition, if the nucleated syrup is discharged onto the conveyor belt too fast, a substantial portion of the syrup can be transformed into an amorphous "sugar glass" with a relatively higher moisture content than the normal desired crystals.
A disadvantage of sugar crystalline transformation processes is the stochastic or random nature of nucleation. Consequently, the control of crystallization in these processes is considered more of an art than a science. These processes are difficult to maintain in equilibrium during continuous production of crystallized sugar.
Another disadvantage with sugar transformation processes is the common requirement for further drying of the crystallized product. Such "post-crystallization drying" is expensive because of the cost of the necessary capital equipment and the use of energy.
Known sugar transformation processes provide unsatisfactory results with the presence of small amounts of foreign matter, including dust particles, entrapped air, or crystals from localized cooling. These conditions can lead to premature nucleation. If a supersaturated syrup is high in soluble impurities, only a few crystals are induced, which then grow to be relatively large in size. When such a feed sugar syrup is introduced into a shearing apparatus, such as an impactor or a colloid mill, the rate of crystallization is slowed. This condition results in the formation of large, relatively wet crystals.